旁系群在支序分类学是指一个演化支上某个单系群最亲近的分类群，又称为姐妹群。如下图所示：

A

B

C

单系群A是单系群B的旁系群，同时B也是A的旁系群，两者从一个最近共同祖先演化而来，而关系较远的C就是它们的外类群。A和B也可以组成一个单系群单元D，此时这个单元D就是C的旁系群，反之亦然……，以此类推，用以概括更远的旁系群。在支序分类学的定义上，A、B、C可以是生物样本、物种或分类单元。在描述物种时，有时会以“姐妹种”（英语：sister species）来称呼。

在种系发生学的亲缘关系研究中使用“旁系群”这个概念时，一般只考虑研究中所列举的样本，例如：在现存物种当中，鳄鱼是鸟类最亲近的旁系群，但这仅仅是对现生种而言；如果将已灭绝的种类算进来，鸟类作为恐龙的直系后代，鸟类和鳄鱼之间还隔着许许多多的已灭绝的旁系近亲。所以，需留意“旁系群”这个概念只是一个相对的概念，仅仅用来描述研究中所列出的类群、物种或样本之间相对的亲疏远近关系。

A sister group or sister taxon is a systematic term from cladistics denoting the closest relatives of a given unit in a phylogenetic tree. The expression is most easily illustrated by the cladogram, where A, B, and C each represents a group:

A

B

C

The sister group to A is B; conversely, the sister group to B is A. Groups A and B, together with all other descendants of their last common ancestor, constitute a clade, here clade AB; the sister group to clade AB is C. The whole clade ABC will itself be rooted in (branched from) the larger tree, which offers yet more sister-group branches that are related but further removed from the leaf nodes, such as A and B. As per cladistic standards, A, B, and C may represent specimens, species, taxon-groups, etc. In cases where they represent species, the term sister species is sometimes used.